Known disc players of the kind described incorporate an optical pickup adapted to be moved linearly in the radial direction of a disc. A pickup is provided with an objective lens which is driven both in the focussing direction and tracking direction thus optically reading an information recorded in the disc. In order to attain a highly precise reading of the information in this type of optical disc player, therefore, it is necessary to conduct both focussing control for controlling the distance between the objective lens and the information track by moving the objective lens such as to eliminate any change in the distance attributable to distortion and oscillation of the disc, and tracking control for moving the objective lens in a manner to follow up the information track. These two types of control necessitate an objective lens driving apparatus capable of driving the objective lens both in the focussing direction and the tracking direction.
A known objective lens driving apparatus, which has been improved to some extent, will be explained hereinunder. FIG. 1 is a perspective view of the known objective lens driving apparatus, while FIG. 2 is an exploded perspective view of the same. An objective lens designated by reference numeral 1 is held by a holder 2. The objective lens 1 is movably supported by web-like supporting members 4 which are made of a plastic material and fixed both to the holder 2 and a base 3 of the objective lens driving apparatus. U-shaped yokes 5a, 5b made of a magnetic material are fixed to the base by means of small screws 6a to 6d. Permanent magnets 7a, 7b are secured to respective yokes 5a, 5b in a manner to provide magnetic gaps between these permanent magnets and the associated yokes. Coils 8a, 8b are wound around the holder 2.
This known objective lens driving apparatus operates in a manner which will be explained hereinunder.
The levels and polarities of electric currents supplied to the coils 8a, 8b are suitably controlled so that the holder 2 is electromagnetically driven by the electromagnetic forces exerted across the magnetic gaps formed between the permanent magnets 7a, 7b and the yokes 5a, 5b thereby moving the objective lens 1 two-dimensionally within a plane as indicated by arrows in FIG. 1. Consequently, the objective lens 1 is moved in two directions, i.e., in the focussing direction and in the tracking direction, thus allowing a high accuracy in the reading of the information.
In this known objective lens driving apparatus, the objective lens is supported by the web-like supporting members 4 which are adapted to be flexed to permit the movement of the objective lens 1 both in the focussing direction and the tracking direction. This arrangement is complicated and raises the cost of production considerably and is liable to allow local partial resonance which impedes the operation of the apparatus tremendously. Namely, when the supporting members 4 are flexed in the focussing direction, stress is produced not only in the focussing direction but also in the tracking direction and an undesirable local resonance is caused by the stress acting in the tracking direction.
In this known objective lens driving apparatus, it is necessary that clearances between the holder 2 and the assemblies of the yokes 5a, 5b and permanent magnets 7a, 7b are controlled highly precisely. Furthermore, since the yokes 5a, 5b and the base 3 are formed as separate parts, there is a fear that the aforementioned gaps may be eliminated due to the sum of the dimensional errors of these parts. Therefore, it is required that these parts be finished within small tolerances, resulting in a raised cost of production of the parts.
The suporting members 4 in support of the objective lens are constructed such that the lens is held horizontally when no load is applied thereto. In the assembled state of the apparatus, therefore, the supporting members are deflected downwardly due to the weight of the lens and the holder, so that these supporting members tend to suffer from a torsional resonance. This problem can be solved by supplying the driving coils with biasing electric currents such as to raise the objective lens by the electromagnetic force produced by the biasing electric currents. This solution, however, causes other problems such as wasteful use of electric power and generation of heat.